1. Field of the Invention
The present general inventive concept relates to an illumination unit having an LED and an image projecting apparatus employing the same.
2. Description of the Related Art
In general, an image projecting apparatus includes an illumination unit to illuminate an optical modulation element. A metal halide lamp or a super high-pressure mercury lamp are typically used as a light source of the illumination unit. Since the metal halide lamp and the super high-pressure mercury lamp are very large, the illumination unit is also large. Additionally, since the life span of these light sources are several thousands of hours at best, the light sources need to be replaced frequently. In an effort to solve these problems, an LED having a relatively longer life span has been used as the light source. For example, U.S. Patent Publication Application No. 2003-0133080 entitled “LED-Illumination-Type DMD Projector and Optical System Thereof” discloses an image projection apparatus using an LED.
However, heat is typically generated when the LED is operated, and an optical efficiency of the LED depends on a temperature thereof. FIG. 1 is a graph illustrating changes in relative optical power of the LED according to changes in temperature. Referring to FIG. 1, when the relative optical power corresponding with 45° C. is assumed to be 100%, the relative optical power increases as the temperature decreases. Similarly, the relative optical power decreases as the temperature increases. Thus, a heat radiating structure which can effectively dissipate heat generated by the LED is required. Korean Patent Publication Application No. 2004-37523 discloses a heat radiating structure including a heat radiating fin that is disposed around the LED in a circular manner.
The amount of light generated by the LED is smaller than that generated by the metal halide lamp or super high-pressure mercury lamp. Thus, in order to obtain an appropriate amount of light, an LED array is used. In particular, an illumination unit of the image projecting apparatus includes a plurality of LEDs emitting lights having different colors. In this case, since more than one heat radiating structure disclosed by Korean Patent Publication Application No. 2004-37523 is applied to each of the LEDs, the size of the illumination unit increases.